Devices utilizing conducting polymers are the subject of intensive research in government, university, and corporate laboratories. A majority of the conducting polymers are produced by aqueous or liquid-based methods. Some conducting polymers are produced by thermal evaporation steps, occasionally combined with photochemical methods. Very few prior methods utilize energetic deposition of ions, and in many cases, the resulting polymer films had insufficient film properties for practical applications.
A wide variety of gas phase ions having kinetic energies of 1 to 107 eV increasingly are being used for the growth and modification of state-of-the-art material interfaces (References 25, 26). Ions can be used to deposit thin films; expose fresh interfaces by sputtering; grow mixed interface layers from ions, ambient neutrals, and/or surface atoms; modify the phases of interfaces; dope trace elements into interface regions; impart specific chemical functionalities to a surface; anneal materials; and create micron- and nanometer-scale interface structures. Ion-induced processes are at the forefront of nanotechnology because they allow engineering of interfaces with specific wetability, hardness, resistance to corrosion, optical parameters, electronic functionality, dimensionality, and/or biocompatibility.
FIG. 1 contains schematic diagrams of several general experimental configurations in which ion surface modification is either central or plays an important role. (a) Direct ion modification involves simply an ion source (IS) to deposit ions (+) on a substrate (S). (b) Mass-selected ion modification filters out ions of a single mass-to-charge ratio for deposition on the surface. Ion beam synthesis, ion beam deposition, ion beam sputtering, ion beam sputter deposition, reactive ion beam etching, and dual ion beam sputtering are all varieties of direct or mass-selected ion modification. (c) Ion beam-assisted deposition simultaneously adds a source of neutral species (°) to deposit additional material or provide a reagent for ion-induced chemistry. Two variants on this method are electron beam ion-assisted deposition and chemical assisted ion beam etching. (d) Magnetron sputtering uses a magnetically confined discharge (M) to sputter ions and neutrals from a target (T) onto S. The discharge here and in plasma processing is established by a direct or alternating voltage difference applied between T and S. (e) Plasma processing uses a gas feed into the chamber to establish the discharge, with ideally no sputtering of the electrode. (f) Pulsed laser deposition employs a pulsed laser (L) to ablate a target material and thereby eject a plume of neutrals and ions for deposition onto S.
The versatility of ion-surface modification places the method at the center of several methods of film preparation, and is the subject of a wide range of fundamental research (References 25–27, 81–92). Not only can interface properties be adjusted experimentally via ion-surface interaction, but experimental data can be supplemented by an array of computational methods that accurately model those interactions (References 25, 28, 29). Ion-surface collisions also play an important role in other interface modification methods that might be considered unrelated at first glance, including plasma processing and pulsed laser deposition (References 25, 30, 31). However, a collision between a specific ion of a given kinetic energy with a surface is more readily modeled by computer simulations than are plasma or laser based methods.
Organic ions in the hyperthermal energy range (1–500 eV) play a critical role in many of the aforementioned energetic deposition processes, especially when organic vapors or polymeric targets are utilized (References 25, 26, 31). Hyperthermal polyatomic organic ion beams are advantageous for practical surface modification due to the unique collision dynamics and the ability to transfer intact chemical functionality to the surface (Reference 32). For example, organic ions often can be soft-landed as intact species upon many surfaces at ≦10 eV collision energies (References 26, 33–35). Polymeric films for applications in optoelectronics can be grown from organic ion sources (Reference 36). Hyperthermal polyatomic organic ion beams also confine their modification to the top few nanometers of a surface, making them particularly useful for nanofabrication (Reference 37).
Oligo- and polythiophenes have been investigated extensively as conducting polymers for use in light-emitting diodes, electrochronic devices, field effect transistors, antistatic coatings, sensor films, organic photovoltaics, and recording materials (References 5, 93, 94). New methods of growing polythiophene films, and other conducting polymers, having desired optoelectronic properties are critical to the success of these various applications.
Methods utilizing polyatomic ion deposition display promise for the production of new types of polythiophene and other classes of conducting polymers. Fluorocarbon and siloxane polymeric films have been deposited directly onto various substrates from mass-selected, gaseous, organic cations with 5–200 eV kinetic energies (References 6–10b). Mass-selected organic cations (>20 eV) have been shown to create selective chemical bonds with self-assembled monolayers (References 11, 12) and carbon nanotubes (Reference 13). Ion-assisted deposition from a non-mass-selected source has produced new conducting polymers (References 14, 15). Atomic ions with 1–100 keV kinetic energies have been used to produce carbonaceous films from gaseous thermal beams of organic compounds (References 9, 16). Kiloelectronvolt atomic ions also have been utilized to modify polymer films (Reference 9), at times with the aim of creating conducting polymers (Reference 17).
Numerous publications discuss the use of ion-assisted deposition in the preparation of organic films. For example, M. J. Vasile et al., Vac. Sci. Technol. B, 7, 1954–1958 (1989) describes an ion-assisted deposition for organic film growth at very high ion kinetic energies (>10 keV). No conducting polymer properties were sought or demonstrated for the organic films produced in this publication.
A. Moliton, “Ion implantation doping of electroactive polymers and device fabrication,” in Handbook of Conducting Polymers, 2nd ed., T. A Skotheim et al., Eds., Marcel Dekker, New York, N.Y., pp. 589–638 (1998), discusses ion deposition and implantation for the production and modification of conducting polymers. For example, this publication describes the modification of a polythiophene film using >10 keV atomic ions. Furthermore, the publication proposes that <1 keV ion-assisted deposition using nonreactive atomic ions also may be utilized for the production of a conducting polymer. The publication does not describe the use of polyatomic ions for this purpose, and no data is provided for any deposition at ion energies <1 keV.
H. Usui, Thin Solid Films, 365, 22–29 (2000) and J.-Y. Kim et al., J. Appl. Phys., 91, 1944–1951 (2002) describe the use of ion beam-assisted deposition to produce thin organic films for conducting polymers and related applications. The publications do not describe the use of polyatomic ions in film growth. The publications also do not describe use of a separate source of chemically distinct organic compounds independent of those used in the ion source to assist film deposition. No polythiophene or thiophene-containing polymers are produced or disclosed.
M. B. J. Wijesundara et al., J. Appl. Phys., 88, 5004–5016 (2000) describes ion sources, surface analysis, and data analysis in the deposition of organic films from mass-selected ions. M. B. J. Wijesundara et al., Langmuir, 17, 5721 (2001) and B. Ni et al., J. Phys. Chem. B. 105, 12719 (2001) discuss the deposition of organic films using mass-selected ions. J. T. Yates, Jr., Experimental Innovations in Surface Science. A Guide to Practical Laboratory Methods and Instruments, Springer-Verlag: New York, Ch. 198 (1998) describes a gas doser used to prepare conducting polymers. However, improved methods that provide more control over film properties than the ion deposition methods described to date still are required.